184 PLANT SOCIOLOGY 



nigricans-Plantago crassifolia association, the Deschampsietum mediae, 

 and the Rosmarinus-Lithospermum fruticosum association of southern 

 France; and all the rock-crevice associations of the Potentillion alliance 

 and related communities of northwestern Africa have been rather 

 minutely investigated. 



Many cryptogamic communities are perhaps even more strictly 

 limited to the occurrence of highly saturated calcium solutions. The 

 lime-encrusting tufa builders, Eucladium verticillatum, Gymnostomum 

 calcareum, Cratoneuron irrigatum, form broad carpets on rocky slopes 

 and walls which are constantly wet with waters rich in lime. The C. 

 commutatum-Arabis belUdifolia association in all its facies is one of the 

 regular phenomena of cold, calcareous springs in the Alps. Motyka 

 (1926) describes a number of lime-constant lichen communities of the 

 Tatra, such as the Lecanora lamarckii association, the Verrucaria 

 calciseda sub-association, and the L. reuteri association. He stresses 

 particularly the unusually sharp line of division between the lime- 

 constant and lime-avoiding lichen associations. Similar communities 

 inhabit the dolomite of the Swabian Alps, such as the V. calciseda 

 community with its several variants of central Germany (Kaiser, 1926) 

 and the Alps. 



That aquatic vegetation, including plankton, reacts sharply to the 

 lime content of the water is very well known. The calciphobous, 

 oligotrophic association of Isoetes echinospora (with /. echmospora, I. 

 lacustris, Suhularia, etc.) of the lakes in the Vosges and Black Forest is 

 entirely missing in the calcareous waters of the northern foothills of 

 the Alps and reappears (without Suhularia) only at the southern slopes 

 of the Alps in the Ticino valley. 



To summarize briefly : The lime-avoiding species are in part such as 

 are harmed or are hindered in their development by saturated lime 

 solutions; in part, acidophilous species, which thrive only upon acid 

 soils with an excess of H ions, and whose absence upon lime is readily 

 explained by their relation to the soil reaction. 



To what extent the indifferent and lime-favoring species can tolerate 

 lime it is at present impossible to state. In one essential the observed 

 facts and experimental investigations tally; the Ca ions (and perhaps 

 Mg ions) seem to be the decisive factor in regard to lime constancy; the 

 direct action of the supply of calcium ions cannot be replaced by the 

 physical properties of the soil. The occurrence of calcicolous (lime- 

 constant) communities is strictly dependent upon solutions rich in 

 calcium. 



Calcium Determination. — Calcium carbonato, being an easily 

 soluble compound, is always accessible to plants. Instead of laborious 



